1. Field of the Invention
The present invention relates to an image processing apparatus for electrically processing an original image such as a facsimile, a digital copier or the like, and more particularly to such image processing apparatus capable of correcting unevenness in the image signals obtained by reading an original image with an image sensor.
2. Description of the Prior Art
There are already known various apparatus for electrical image processing such as facsimile or copier, and, in most of such apparatus, an original image is illuminated by a light source and the resulting reflected light is read by an image sensor such as a charge-coupled device (CCD). Uniform image signals cannot often been obtained, however, because of uneven brightness of the light source such as fluorescent lamp over the entire length thereof, uneven light transmission of the lens resulting from vignetting of the lens, and uneven sensitivity of the image sensor. In the following description, the unevenness in the image signals will be called shading error. For the purpose of obtaining satisfactory image signals, there has been proposed a device for electrically correcting said shading error.
FIG. 1 shows an example of conventional image processing apparatus capable of electrical shading correction.
In FIG. 1, the light reflected from an original image illuminated by a light source is focused, through an unrepresented optical system composed of lenses, prisms etc., onto a CCD sensor 1 functioning as a photoelectric converting element. The CCD sensor 1 is composed of a linear array of plural photosensitive elements, each of which reads a pixel of the image.
The timing of image data reading from the CCD sensor 1 is controlled by a read controller 7. The output signals of the CCD sensor 1 are amplified to a determined level by an amplifier 3 and then binary digitized by a comparator 11. The shading error is corrected by regulating the reference voltage of said comparator 11.
The reference voltage of the comparator 11 is determined according to shading error data stored in a memory 9, composed for example of a random access memory. More specifically, the read controller 7 coordinates the data output from the CCD sensor 1 with the read-out of the shading error data from the memory 9 to regulate the reference voltage for binary encoding in the comparator 11 through an A/D, D/A converter 5 in relation to the image reading position of the CCD sensor 1, thereby correcting the shading error in response for example to the intensity distribution of the light source.
The shading error data are entered by scanning a reference member such as a standard white board with the CCD sensor 1, prior to the reading of the original image. The analog output signals obtained from the CCD sensor 1 by reading said reference member are converted, by the A/D-D/A converter 5, into digital signals of plural bits per pixel, and such digital signal for example of a line are stored in the memory 9 in relation to the image reading position.
In the above-described image processing apparatus, high-speed A/D and D/A converters are required for achieving a higher image reading speed. In general, such high-speed A/D and D/A converters are complex in structure and expensive, and increase the cost of the entire image processing apparatus.
Also, the memory for storing the shading distortion data is another factor of cost increase, since it requires a considerably large capacity for storing the digital data of an entire line, including plural bits for each pixel.